The preparation of rubber-modified styrene resins of good impact resistance requires an exercise of proper control over the particle diameter and particle diameter distribution of rubber dispersed in the resins.
The rubber-modified styrene resins tend to improve in appearance but suffer a loss in impact resistance as the rubber particle diameter diminishes and the optimal particle diameter is generally said to be in the range from 1 to 5 .mu.m, preferably 1.5 to 4 .mu.m.
On the other hand, the impact resistance of rubber-modified styrene resins is closely related to the content of styrene polymers occluded in the rubber particles and is said to improve with the increasing content of the occluded styrene polymers.
It is therefore advantageous for the improvement of the impact strength and other mechanical strengths of rubber-modified styrene resins to increase the rubber particle diameter without adversely affecting the appearance of the resins and, at the same time, to increase the content of styrene polymers occluded in the rubber particles.
The present inventors previously disclosed a process for preparing rubber-modified styrene resins which comprises carrying out preliminary polymerization in two complete mixing type reactors, until before the rubber phase inversion in the first reactor and until after the rubber phase inversion in the second reactor, and completing the polymerization of the effluent from the preliminary polymerization in a plug flow type reactor in Japan Tokkyo Kokai Koho No. 63-118,315 (1988) and proposed to bring the rubber particle diameter and the occluded polystyrene content in an optimal range by controlling the solid content in the aforesaid first reactor at a relatively high level, the solid content in the second reactor at a relatively low level, and the difference of the two solid contents in a specified range.
A process for polymerization is also proposed in Japan Tokkyo Kokai Koho No. 63-118,346 (1988) which measures the viscosity of the effluents from the first and second reactors and controls the ratio of the two viscosities within a minimal range, say, 2 to 3.
The polymerization according to these processes is carried out until immediately before the phase inversion in the first reactor, at a conversion kept as low as possible in the second reactor, and until completion in the following reactor and this mode of operation has proved itself capable of yielding products with a high occluded polystyrene content and markedly improved mechanical strength.
The problem confronting the adoption of a process such as above is, as pointed out in the past, that the rubber particles diminish in diameter after the phase inversion. This phenomenon is more pronounced when a peroxide is added as polymerization initiator for the purpose of increasing the amount of polystyrene captured in rubber during the initial phase of the reaction. The likely explanation is that more graft polymers form between rubber particles and styrene polymers on addition of a peroxide to increase the content of styrene polymers occluded in rubber particles, but the graft polymers thus formed act as surfactant between the rubber particles and the styrene polymer continuous phase to produce a finer dispersion of rubber particles and an excessive reduction in particle diameter.
A process developed as a solution to this problem is based either on decreasing the addition of a polymerization initiator at the start of polymerization to such a level as to allow control of the rubber particle diameter in the range from 1.5 to 4 .mu.m or on carrying out the polymerization in the complete absence of an initiator. Such a process surely reduces the formation of graft polymers and helps to increase the rubber particle diameter, but it does not increase the content of styrene polymers occluded in the rubber particles, a property of great importance, to a satisfactory level and hence is unable to accomplish the anticipated property improvement.
Another process for increasing the rubber particle diameter is based on addition of a chain transfer agent in large amounts. It also suppresses grafting and increases the particle diameter, but does not bring the content of occluded styrene polymers to a satisfactory level and hence does not bring about satisfactory properties.
An attempt has been made to solve the aforesaid problem by control of the reactor-related factors, namely, to form rubber particles without excessive size reduction by as gentle agitation as possible in the second reactor. In brief, the intention here is to minimize the shear energy imparted to the rubber particles by reducing the speed of rotation of agitator blades attached to the reactor. Reduction in the agitation speed, however, decreases the fluidity of the polymer solution inside the reactor. As a result, the particle diameter distribution broadens excessively even when the average rubber particle diameter is brought into the desired range, leading to insufficient manifestation of the desired property-improving effect and production of low-strength polymers.
The present inventors undertook extensive studies with the objective of promoting the graft reaction in the initial phase of polymerization to increase the content of occluded styrene polymers after the phase inversion and, at the same time, producing polymers having an optimal average particle diameter and an optimal particle diameter distribution of rubber, found the following, and completed this invention.
In a polymerization process wherein a feed containing a polymerization initiator is polymerized preliminarily in two complete mixing type reactors at a conversion of as high as possible in the first reactor and at a conversion of as low as possible in the second reactor and then polymerized mainly in a following reactor with the conversion raised to a specified level, the aforesaid problem can be solved by using rubbers which are commercially unavailable so far and show an extremely high viscosity in styrene solution. More desirable results are obtained by using such rubbers selectively while varying the solution viscosity with the rubber content in the feed.